Duplex perception is a laboratory phenomenon in which one stimulus, or one stimulus component, simultaneously contributes to two distinct percepts in human listeners. The primary demonstrations of this phenomenon involve altering the quality of a critical component of a speech syllable (/da/ or /ga/) so that the component is heard out as a separate, nonspeech event (a chirp) while also contributing to the perception of the syllable. The important question is whether this demonstration reflects a specialized mode for processing speech information, or a generalized perceptual tendency in the auditory system for attributing stimuli to a particular sound source. Many speech perception phenomena can be replicated in nonhuman primates. The perceptual abilities that monkeys demonstrate cannot be attributed to uniquely human speech processing mechanisms and may be due to more general auditory mechanisms. Conversely, perceptual tasks on which human listeners succeed and monkeys fail are good potential candidates for special-processing mechanisms in humans. In 1997, we conducted Duplex Perception tests with nonhuman primates. These experiments used synthetic /da/ and /ga/ syllables as stimuli. Tests of Duplex Perception are valuable for theory construction in that they provide a critical test of the two opposing models of the initial status of the mechanisms underlying speech processing in humans.